Walls are perimeter work, not floor-area work
Gross wall area is perimeter times ceiling height. Perimeter is 2 × (length + width) for a simple rectangle. Two rooms with the same floor footprint need different paint if one has 8 ft ceilings and the other has 10 ft — easy to forget when someone quotes “a 12 by 12 room.”
Subtract doors and windows you will not paint as wall. The Paint Calculator uses planning deductions of about 21 ft² (roughly 2 m²) per door and 15 ft² (roughly 1.4 m²) per window. Huge patio doors deserve a custom subtraction; tiny windows often are not worth nitpicking if you are already rounding up a tin.
Closets painted the same colour belong in the takeoff. Unpainted closets or a different product should be separate — write which is which on the sketch.
Coats, primer, and real coverage
Can labels assume a recommended spread rate on a suitable surface. A common planning rate is about 350 ft² per gallon (around 12 m² per litre) per coat on smooth, sealed walls. Texture, thirsty new drywall, and cheap paint all reduce coverage. Dark-to-light changes often need more film than a same-family refresh.
Two finish coats are the default for a noticeable colour change. One coat can work for a similar-colour refresh with a high-hide product on sound paint — plan two unless you have tested a patch.
New plaster or drywall usually wants a sealing primer before finish colour. Treat primer as its own pass in the Paint Calculator. Skipping it to “save a tin” is how flashing shows under angled light.
Ceilings, trim, and doors are different jobs
Ceiling paint uses floor area (length × width), not the wall formula. Many people underestimate ceilings because they shop only from a wall tin count. Keep a different colour or sheen on its own line.
Trim, skirtings, and doors usually take a different sheen and often a different base. Do not fold trim into the wall gallon count. Budget a dedicated tin — running out of enamel mid-cut-in is as painful as running out of wall paint.
Sample pots decide colour in your light. They are not a substitute for a full-room calculation once the colour is chosen.
US gallons vs UK litres
US projects usually buy gallons and quarts; UK projects buy litres and 2.5- or 5-litre tins. The Paint Calculator works in either system — match the output to the shelf you will shop.
Always round up to whole containers. Keeping a labelled leftover for touch-ups is a reason to round up slightly — not a reason to buy an extra 5-litre tub for a small powder room.
Batch codes matter on large open-plan walls. Buy enough in one go for continuous surfaces you will see together. A second purchase weeks later can sit in a different batch even with the same colour name.
Surface condition changes the tin count
Glossy existing paint, stains, and repairs affect prep and sometimes coat count. Stain-blockers and patch primers are extra products — budget them when you can see the problem.
If you are hanging or patching board first, finish that work and prime before you trust a final paint quantity. After drywall, reuse wall and ceiling areas in the Paint Calculator so boarding and paint takeoffs stay consistent.
Heavy texture drinks paint. Drop your effective coverage rate rather than pretending the label’s best-case number still applies. One extra tin beats a starved roller patch.
Worked example
A 12 by 10 ft room with 8 ft ceilings, one door, and two windows, two coats: perimeter 44 ft, gross walls 352 ft². Deduct about 21 + 15 + 15 = 51 ft². Net walls ≈ 301 ft². Two coats → about 602 ft². At 350 ft² per gallon that is roughly 1.7 gallons — buy 2 gallons (or the matching UK litre tin size).
Add the ceiling separately if you are painting it: 120 ft² times coats, divided by coverage. Trim is a third line. Price volume with the Paint Cost Calculator or Cost Estimator when you are building a room budget, not only a shopping list.
Run the same dimensions through the Paint Calculator. If your notepad and the tool disagree, you probably forgot a coat, mixed floor area into walls, or skipped openings on one method.
Buying and storing without waste theatre
Order for the rooms you will paint in one campaign. Stopping for months mid-colour across an open stair void invites a batch mismatch. If the project is phased, buy continuous surfaces together.
Store leftover paint sealed, labelled with room and date, away from freezing. Touch-ups work best with the same sheen and a feathered edge.
Disposal rules differ by locality. Do not pour leftover paint down drains. Factor a small disposal cost into big projects if you will have partial tins you cannot keep.
Use a calculator for this
Frequently asked questions
Can I paint a room with one coat?
Sometimes, when refreshing a similar colour with a high-hide product on a sound, sealed surface. For a clear colour change, new board, or stained walls, plan on primer as needed plus two finish coats.
Should leftover paint be part of the estimate?
Rounding up to the next tin for touch-ups is sensible. Buying a whole extra large tub “just in case” for a small room usually wastes money. Label what you keep.
Do I measure in feet or metres?
Use the unit system your paint is sold in. The Paint Calculator supports both — stay consistent from wall measure to tin coverage so you do not convert twice by accident.
How do ceilings change the total?
Ceilings use floor area times coats, separate from walls. Ignoring the ceiling under-counts whenever you intend to repaint it the same weekend as the walls.
What coverage rate should I assume?
Start near 350 ft² per gallon (about 12 m² per litre) per coat on smooth sealed walls, then reduce for texture, bare board, or deep colour changes. When unsure, round up a container.
Where does paint cost fit beside litres?
Once you know volume, price tins with the Paint Cost Calculator or fold material lines into the Cost Estimator. Quantity first, money second — reversing that order produces neat budgets that still run out of paint.